1. Field of the Invention
The present invention relates to a transmitting device of a wireless orthogonal frequency division multiplex (OFDM) communication system.
2. Description of the Related Art
In wireless telecommunication, the transmission quality between a transmitting device, such as a base station, and a receiving device, such as a mobile terminal, depends strongly on the respective transmission environment and is often deteriorated by fading effects and the like. This often leads to poor speech and data transmission quality, particularly if only one single antenna is used on the transmission side and one single antenna is used on the receiving side. Therefore, some mobile terminals for wireless telecommunication systems, such as the GSM system, comprise two or more and different kinds of antennas built as internal or external antennas in the mobile terminal. However, it is desirable that modern mobile terminals are as small and light as possible and therefore it is an increasing interest to use only a single antenna in these mobile terminals. In order to allow the use of only a single antenna on the receiving side, particularly the mobile terminal side, it has been proposed to use more than one antenna on the transmitting side, particularly the base station side, so that the diversity gain can be used for a better transmission quality. This scheme is called transmit diversity. Transmit diversity generally means that more than one antenna, e.g. two antennas, transmit data simultaneously to a receiving device. If the same data are transmitted in parallel by two antennas, the receiving side has a chance to receive signals at least from one of the antennas with an acceptable transmission quality so that a good connection can be ensured. One specific approach in the transmit diversity scheme is the use of a so-called space time coding. The resulting space time transmit diversity (STTD) has been adapted and is part of the UMTS standard for the next generation of mobile telecommunication.
In a space time transmit diversity system, a transmitting device, such as a base station, comprises e.g. two antennas arranged spaced apart from each other in a space diversity arrangement. A stream of data to be transmitted to a receiving device, such as a mobile terminal, is encoded and processed so that two parallel data streams are generated. After further processing corresponding to the respective wireless communication system, the data of each of the two data streams are transmitted by a respective one of the two antennas. Although generally the same data content is transmitted by each of the two antennas, the signals transmitted by the two antennas are not absolutely identical, but data symbols to be transmitted are mapped or coded slightly differently on the signals transmitted by each of the antennas. This allows a receiving device receiving the signals transmitted from the two antennas with only a single antenna to distinguish and separate signals coming from one of the transmitting antennas from signals coming from the other of the transmitting antennas. Since the two transmitting antennas are arranged in a space diversity arrangement, cross interference is avoided and the receiving device can then, after a corresponding channel estimation, distinguish and combine the signals from the two transmitting antennas to obtain a better transmission quality. The channel estimation in the receiving device is usually performed on the basis of pilot symbols transmitted from the transmitting device. The receiving device performs a channel estimation by comparing received pilot symbols with an expected pilot symbol to measure the channel response and to tune the receiving device to the best transmission channel, i.e. the transmitting antenna to which the better connection exists.
The above-mentioned UMTS system bases on a code division multiple access (CDMA) scheme. The CDMA scheme is only one of several possible multiple access schemes used in wireless telecommunication. For wireless telecommunication with high data rates, the orthogonal frequency division multiplex (OFDM) scheme is known, in which the available frequency band used for a communication is divided in a plurality of frequency subcarriers, whereby adjacent frequency subcarriers are respectively orthogonal to each other.